Iain Duncan Smith's flagship welfare reform programme, under which most benefits will be rolled into one universal credit, is heading for a disaster, according to the government's poverty tsar, Frank Field.

As MPs prepare to vote on the reforms in the Commons on Tuesday, Field says the universal credit is "practically unachievable" and will "rot the soul" of low-paid workers by encouraging dependency.

Field, a Labour MP and former welfare reform minister, has previously been well disposed towards some Tory welfare reforms. In an article for the Guardian, he writes that Duncan Smith was brave to resist David Cameron's attempt to move him to the Ministry of Justice in last week's cabinet reshuffle.

But Field writes: "It was the prime minister's wish to avert a catastrophe that drove him to try to move his work and pensions secretary so that the government could quietly shut down the whole reform. His failure to act leaves the disaster on course."

Cameron tried to move Duncan Smith from the Department for Work and Pensions amid fears in the Treasury that the costs of the universal credit, due to be introduced in stages between next April and 2017, are spiralling out of control.

The new system is designed to simplify benefits and increase incentives to work. Under the current system, claimants can lose 90p of every extra £1 they earn as their benefits are tapered off. Under the universal credit this will fall to 65p in every £1.

On Monday in the Commons, Duncan Smith pledged to press ahead with the universal credit, though he did offer a concession in one area highlighted by Field and other critics. He said he would consider making payments to the lowest-paid on a fortnightly basis, as happens at present, rather than monthly as planned.

Field dismisses the entire reform and rejects one of Duncan Smith's central arguments: that the universal credit will mark a step away from means testing. He writes: "Means testing only encourages dependency, and the universal credit is, in one sense, the ultimate form of means testing.

"It obviously gets extra money to hard-working families who earn low wages, but in doing so it rots the soul. Recipients have to be saints not to take the loss of credit payments into account when deciding whether to work longer or to train for a more highly paid job."

He says he has asked the National Audit Office to carry out a special inquiry into the implementation of the universal credit, amid fears that the IT changes are too ambitious. Field writes: "I am, therefore, against universal credit in principle, but I also fear that the programme is practically unachievable."

Duncan Smith told MPs on Monday: "Universal credit is on time, on budget and I particularly want to stay here to see that through."